Carbon monoxide comes first
Carbon monoxide (CO) is a gas you cannot see or smell, and it can come from any fuel-burning appliance that is not venting properly. Working CO alarms are your protection. Ontario's Fire Code requires them in homes with a fuel-burning appliance, a fireplace or an attached garage, next to each sleeping area and, since January 1, 2026, on every storey, so check your local requirements. Test the alarms regularly and replace them before the end-of-life date printed on the unit.
If a CO alarm sounds, get everyone out of the home, including pets, and call 911 from outside. If you smell gas, the same applies: leave, then call your gas utility's emergency line or 911 from outside. Do not turn lights or appliances on or off on the way out, do not use a phone inside, and do not go back in until you have been told it is safe.
Do not try to find the source yourself while an alarm is sounding. Leave first, then call.
Sounds that deserve attention
Furnaces make ordinary sounds at start-up and shutdown. A new or changed sound is the point to pay attention to. Causes vary, so treat this as a guide to urgency and not a diagnosis.
Banging or popping at start-up
Can point to delayed ignition or expanding ductwork. Delayed ignition is worth a prompt look, since gas should light cleanly.
Squealing or grinding
Often a blower or inducer motor bearing on its way out. Grinding that continues is a reason to switch the furnace off and call.
Rattling or whistling
A loose panel or duct is a common cause. Whistling at the grilles can also mean a dirty filter or restricted return.
Repeated clicking without heat
The furnace may be trying to light and failing, then locking out. A resettable fault that keeps coming back should be diagnosed, not reset again and again.
Short cycling, cold rooms and no heat at all
Short cycling means the furnace starts, runs briefly and shuts down, over and over. Common causes include a clogged filter, restricted returns, an overheating limit switch, a dirty flame sensor, a venting problem or a thermostat in a poor location. An oversized furnace can also short cycle because it satisfies the thermostat too quickly.
Rooms that never match the thermostat usually point to airflow, not the burner. Closed or blocked registers, undersized returns, leaky ducts and basement or addition spaces served by the same system all play a part. If one room is always cold, check the registers first, then call us about balancing or return air.
A furnace that will not start at all may have a thermostat, power or safety switch issue, a blocked condensate drain tripping a pressure switch, or a control lockout. The safe checks are below.
Yellow flames, soot and smells: stop and call
A healthy burner flame is mostly steady and blue. Yellow, orange or lazy flickering flames, soot around the burners or on the furnace cabinet, scorch marks, or a persistent burning smell can signal incomplete combustion or a venting problem. These are not do-it-yourself items.
Turn the furnace off at the thermostat, ventilate if it is safe to do so, and arrange a professional inspection before using it again. If a CO alarm has sounded, or anyone feels unwell with the furnace running, treat it as an emergency and leave the home. Water pooling around the furnace is a separate issue, usually a blocked condensate line, and should also be dealt with promptly.
Safe things to check before you call
- Thermostat mode, set point and batteries
- The furnace power switch, and that the lower front panel of the furnace is fully seated
- The air filter, replaced with the right size and the arrow pointing toward the furnace
- Supply registers and return grilles are open and not blocked by furniture or rugs
- Outdoor sidewall vent and intake pipes are clear of snow, ice or leaves; clear them gently by hand
- One reset only if the manufacturer's label instructs it
Do not remove panels, bypass safety switches, relight pilots repeatedly or attempt gas work. Those steps belong to a qualified professional.
When to call, and what helps us help you
Call promptly for repeated lockouts, burner or flame concerns, grinding noises, water around the furnace, or any heating problem during cold weather. See furnace repair for what the service covers. If you have no heat in cold weather, see emergency heating and cooling; emergency service is subject to availability. A diagnostic fee may apply; confirmed at booking.
It helps to have the furnace model and serial number, a description of the sound or symptom, when it started, any blinking fault code, and the date of the last filter change. If the furnace is old, or repairs are piling up, our Furnace Replacement Guide explains when replacement starts to make sense. A yearly visit under furnace maintenance is a good way to catch small issues before they become cold-night problems.